The form of the verb in both the situations depends on the context. However, these are my recommendations:
If it can be deduced from the context (if there is one) that the actions are certainly going to happen the way described in the sentence, you should use to be going to. Anyways, I think you should choose this form for both of them, because there are certain details (the date and the place) specified in the body of the sentence:
This office is going to reopen...
The 2.35 to Bristol is going to leave...
In this context, both the forms are nearly equivalent, but the details are most likely the "decision-makers". However, both of them are equally acceptable.
However, if you would not be constrained to use these forms, and there is no context from which you can tell which one to use, I think you'd better off with:
- This office is reopening on January 2nd. (present continuous - expresses plans)
When it comes to the second sentence, I would use is going to leave, simply because travelling is (usually) carefully planned out and this form of the verb expresses a certain action.
If the statements are written on signs, choose will, because it is shorter and matches such a context.